In continuous casting of steel, in a final stage of solidification, a suction flow of non-solidified molten steel (also referred to as “non-solidification layer” when necessary) is generated in the drawing direction of a cast slab along with solidification shrinkage. In the non-solidification layer, solute elements such as carbon (C), phosphorus (P), sulfur (S), manganese (Mn) and the like are concentrated, and the so-called center segregation is generated when concentrated molten steel flows into a center portion of a cast slab and is solidified.
The center segregation deteriorates quality of a steel product, particularly a thick plate. For example, in a line pipe material used for transporting petroleum or for transporting a natural gas, stress corrosion cracking is generated with the center segregation as an initiation point due to an action of a sour gas. Further, the similar drawbacks occur also with respect to an offshore structure, a storage tank, an oil tank and the like. Recently, it is often the case where the use of a steel product in a harsh environment such as a lower temperature environment or a more corrosion environment is required and hence, the reduction of the center segregation in a cast slab has been considered as a crucial task.
Many countermeasures for reducing the center segregation of a cast slab have been proposed. Among these countermeasures, it has been known that, in a continuous casting machine, a solidification last-stage soft rolling reduction method which performs rolling reduction of a cast slab having a non-solidification layer in the inside thereof is effective. The solidification last-stage soft rolling reduction method is a method where reduction rolls are arranged in the vicinity of a solidification completion position of a cast slab, the cast slab is gradually reduced by rolling with a rolling reduction amount corresponding to a solidification shrinkage amount by reduction rolls and hence, the formation of pores and the flow of concentrated molten steel in a cast slab center portion is prevented whereby the center segregation of the cast slab is suppressed.
In the continuous casting of steel, at the time of exchanging a ladle which is arranged above a tundish of a continuous casting machine and in which molten steel is accommodated (so-called a ladle exchange at the time of performing consecutive continuous casting) or at the time of detecting temperature abnormality in the inside of a mold or the like, there may be a case where it is necessary to lower a drawing speed of a cast slab. In this case, to restore a target speed again, it is necessary to increase the drawing speed. In the solidification last-stage soft rolling reduction method, a specified portion in the vicinity of a solidification completion position of a cast slab during continuous casting is constantly reduced by rolling and hence, it is desirable that the solidification completion position not be changed during continuous casting. However, as described previously, when the drawing speed of the cast slab is changed, there is a possibility that the solidification completion position is changed.
In view of the above, there has been proposed a method in patent literature 1 where, in a continuous casting method, when a drawing speed (casting speed) of a cast slab is changed, aiming at an accurate control of a solidification completion position, a response model which expresses the relationship of a moving response of a solidification completion position of a cast slab with respect to a change in a casting speed and/or an amount of cooling water is prepared, and a manipulated variable of the casting speed and/or the amount of cooling water is calculated based on the prepared response model, and the solidification completion position is controlled.